A liquid developer that is used in electrophoretic printing is used as an example of such a dispersion. This has as particles toner particles that are dispersed in a carrier fluid as a fluid. In the following the preferred embodiment is essentially explained using a liquid developer, without therefore limiting the preferred embodiment to liquid developer.
For single-color or multicolor printing of a printing substrate (for example a single sheet or a belt-shaped recording material), it is known to generate image-dependent charge images on a charge image carrier, which charge images correspond to the images to be printed (comprising regions to be inked and regions that are not to be inked). The regions of the charge images that are to be inked are revealed as toner images on the charge image carrier via toner particles with a developer station. The toner image that is thereby generated is transported via a transfer station to the printing substrate and transfer-printed onto the printing substrate at a transfer printing point. The toner images are fixed on the printing substrate in a fixing station.
A liquid developer having at least charged toner particles and carrier fluid can be used to ink the charge images. A method for such an electrophoretic printing in digital printing systems is known from U.S. 2006/0150836 A1 or U.S. 2008/279597 A1, for example. After the charge images of the images to be printed have been generated on the charge image carrier, these are inked with toner particles into toner images via a developer station. A carrier fluid including a silicone oil as a liquid developer with color particles (toner particles) dispersed therein is thereby used here. The supply of the liquid developer to the charge image carrier can take place via a developer roller that is supplied with liquid developer from a reservoir with liquid developer. The image film generated in the development on the charge image carrier is subsequently accepted by the charge image carrier via a transfer unit and transferred onto the printing substrate in a transfer printing zone.
In this printing method, using the liquid developer the process of electrophoresis is employed to transfer toner particles to the printing substrate in the carrier fluid. The solid, electrically charged toner particles thereby migrate through the carrier fluid as a transport medium, wherein the transport can be controlled via an electrical field between the transfer roller and the printing substrate. In addition to the toner particle charge and the electrical field, the provision of a sufficiently thick carrier fluid layer through which the toner particles can migrate and a sufficient concentration of the toner particles in the carrier fluid are a requirement for this.
The liquid developer used in the printing apparatus can be mixed together in the developer station (in a mixing unit, for example) from a toner concentrate (comprising toner and carrier fluid) carrier fluid. For a trouble-free print image it is necessary that sufficient toner particles are included in the liquid developer, and thus the toner mass concentration in the liquid developer has the provided value. It must thereby be taken into account that, in the printing operation, liquid developer is removed from the mixing unit and is partially applied to the printing substrate.
A defined toner mass concentration and electrophoretic mobility of the toner particles in the carrier fluid is required for a successful and uninterrupted development of the charge images.
The adjustment of the toner mass concentration and mobility of the toner particles in the carrier fluid requires that the toner mass concentration and the mobility of the toner particles can be determined in the developer station. Given a relevant toner mass concentration (for example in the range of 2% to 40% of the liquid developer), electroacoustic methods for the determination of the electrophoretic mobility of toner particles are known that, however, assume a precise knowledge of the toner mass concentration.
From U.S. 2011/058838 A1 a method is known according to which the toner mass concentration in a liquid developer can be determined. For this the liquid developer is charged with at least one ultrasound wave. It is thereby assumed that the sound velocity of the sound propagating in the liquid developer essentially depends on the proportion of the toner particles in the carrier fluid within predetermined temperature limits and constant carrier fluid. The delay of an ultrasound wave in the liquid developer is accordingly measured along a predetermined measurement path, and the sound velocity—which is a measure of the toner mass concentration in the liquid developer—can be determined from this. By measuring the delay of the sound wave in the liquid developer, its toner mass concentration can thus be determined. Given a plurality of liquid developers with known toner mass concentrations, the correlation between the delay of an ultrasound wave and the toner mass concentration can be determined via calibration processes under consideration of the temperature of the liquid developer, and the determined values with regard to delay and toner mass concentration can be stored in a table, for example. By measuring the delay of a sound wave through a liquid developer, this table can be used in order to determine its toner mass concentration. If necessary an interpolation can be made between the values in the table. Comparable methods to determine the mass concentration in dispersions are known from U.S. Pat. Nos. 6,817,229 B2, 5,121,629 A or 7,764,891 B2, for example.
A measurement apparatus to determine the electrophoretic mobility of electrically charged particles in a fluid is known from U.S. Pat. No. 5,245,290 A. A dispersion to be tested that includes electrically charged particles whose mobility should be established is contained in a measurement cell. An alternating electrical field that excites the particles in the fluid to oscillate is applied to the measurement cell. The oscillating particles generate sound waves whose velocity can be assessed. The electrophoretic mobility of the particles can be concluded from the electrical field and the average velocity of the particles in the fluid. A formula to calculate the dynamic mobility of particles in a dispersion can be learned from R. W. O'Brien et al./Colloids and Surfaces A: Physiochem. Eng. Aspects 218 (2003) P. 89-101.
In the known measurement methods, either the mass concentration or the electrophoretic mobility of particles is measured in various measurement cells and with various sample volumes, wherein concentration and temperature differences lead to a reduced measurement precision.